Empirical mode decomposition analysis of random processes in the solar atmosphere
Empirical mode decomposition analysis of random processes in the solar atmosphere
复制标题
DOI：
10.1051/0004-6361/201628306
复制
发表时间：
2016-08
影响因子：
6.5
通讯作者：
D. Kolotkov;S. Anfinogentov;V. Nakariakov
中科院分区：
文献类型：
--
作者：
D. Kolotkov;S. Anfinogentov;V. Nakariakov
上下文。在各种类型的太阳信号中，经常显示出具有幂函数谱能量分布的有色噪声分量。这种频率相关的噪声可能表明太阳大气中各种随机分布的动力过程的运行。目标。我们开发了在有色噪声存在的情况下正确使用经验模式分解(EMD)技术的配方，允许清楚地区分太阳大气中的准周期振荡现象和叠加的随机背景过程。为了说明这一点，我们统计地研究了利用SDO/AIA观测到的太阳大气的日冕层(171A)、色球层(304A)和光球层(1600A)的极端紫外线(EUV)发射强度的变化，这些层来自安静的太阳和太阳黑子本影区。方法：研究方法。EMD因其自适应特性和对处理非平稳和调幅时间序列的基本适用性而被用于分析。为了与EMD的结果进行比较，我们使用了傅里叶变换技术作为标准具。结果。我们经验地揭示了EMD中合成有色噪声的统计特性，并提出了一种从EMD获得的真实信号的本征模式中检测噪声分量的方案。对太阳EUV信号的应用表明，它们确实表现出随机行为，可以表示为不同颜色噪声的组合，其特征是其光谱能量分布中的幂定律指数的特定值。另一方面，在被分析的太阳黑子中探测到的3分钟振荡的能量明显高于相应的噪声水平。结论。在使用EMD分析太阳大气中的振荡时，正确考虑与背景频率相关的随机过程是至关重要的。对于安静的太阳区域，幂定律指数随着光球上方高度的增加而增加，这表明高频过程被困在安静的太阳大气中得更深。相比之下，太阳黑子本影级别较低的特点是幂定律指数较高，这意味着太阳黑子大气深处的较低频率占主导地位。EMD结果与傅立叶变换结果的比较表明，EMD结果具有很好的一致性，证明了EMD的适用性。
Context. Coloured noisy components with a power law spectral energy distribution are often shown to appear in solar signals of various types. Such a frequency-dependent noise may indicate the operation of various randomly distributed dynamical processes in the solar atmosphere. Aims. We develop a recipe for the correct usage of the empirical mode decomposition (EMD) technique in the presence of coloured noise, allowing for clear distinguishing between quasi-periodic oscillatory phenomena in the solar atmosphere and superimposed random background processes. For illustration, we statistically investigate extreme ultraviolet (EUV) emission intensity variations observed with SDO/AIA in the coronal (171 A), chromospheric (304 A), and upper photospheric (1600 A) layers of the solar atmosphere, from a quiet sun and a sunspot umbrae region. Methods. EMD has been used for analysis because of its adaptive nature and essential applicability to the processing non-stationary and amplitude-modulated time series. For the comparison of the results obtained with EMD, we use the Fourier transform technique as an etalon. Results. We empirically revealed statistical properties of synthetic coloured noises in EMD, and suggested a scheme that allows for the detection of noisy components among the intrinsic modes obtained with EMD in real signals. Application of the method to the solar EUV signals showed that they indeed behave randomly and could be represented as a combination of different coloured noises characterised by a specific value of the power law indices in their spectral energy distributions. On the other hand, 3-min oscillations in the analysed sunspot were detected to have energies significantly above the corresponding noise level. Conclusions. The correct accounting for the background frequency-dependent random processes is essential when using EMD for analysis of oscillations in the solar atmosphere. For the quiet sun region the power law index was found to increase with height above the photosphere, indicating that the higher frequency processes are trapped deeper in the quiet sun atmosphere. In contrast, lower levels of the sunspot umbrae were found to be characterised by higher values of the power law index, meaning the domination of lower frequencies deep inside the sunspot atmosphere. Comparison of the EMD results with those obtained with the Fourier transform showed good consistency, justifying the applicability of EMD.